If existing migrations have already been applied, Flyway will by default
validate all of them before migrating.
Our Ansible playbook however only downloads the delta of migrations to
run, not the full history: so if there are existing migrations, the
playbook would fail.
Flyway has a flag ignoreMissingMigrations since 4.1.0, but our binary is
still based on 4.0 and upgrading it is quite hard unfortunately.
Luckily validateOnMigrate skips that validation as well, so it offers a good
workaround for now.
Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>